Polycarbonate

Polycarbonate, typically referring to polycarbonate of bisphenol A, are a clear, strong thermoplastic polymer often used in windows. General Electric's trade name for the material is Lexan. They are easily worked, molded, and thermoformed; as such, these plastics are very widely used in the modern chemical industry. Their interesting features (temperature resistance, impact resistance and optical properties) position them between commodity plastics and engineering plastics. The material is similar to polymethyl methacrylate (PMMA, acrylic, PMMA/Plexiglass), but is significantly stronger, in addition to being more expensive. However, polycarbonate is flexible and can be worked at room temperature without breaking and cracking.

Technical Information

Polycarbonates specifically are simply polymers containing carbonate groups: <math>(-O-(C=O)-O-)</math>. Most commercial varieties are made from rigid monomers, which gifts them with good temperature resistance, impact resistance, and optical properties. Typically polycarbonates are specifically polycarbonates of bisphenol A, such as Lexan.

Polycarbonate of bisphenol A has a melting temperature of <math>267^\circ C</math>, class transition temperature of <math>150^\circ C</math>, upper working temperature of <math>130^\circ C</math>, and lower working temperature of <math>-40^\circ C</math>.

The same material has a density of <math>1.20–1.22 g/cm^3</math>, Rockwell hardness of M70, Young's Modulus of 2.2 GPa, and tensile strength of about 55-75 MPa.

Production

The majority of polycarbonate is manufactured through the reaction of bisphenol A with phosgene <math>(COCL_2)</math>. The bisphenol A is first treated with sodium hydroxide <math>(NaOH)</math> to deprotonate the hydroxyl groups of the bisphenol A:

<math>(HOC_6H_4)2CMe_2 + 2 NaOH → (NaOC_6H_4)2CMe_2 + 2 H_2O</math>

The diphenoxide <math>((NaOC_6H_4)2CMe_2)</math> reacts with the phosgene to create chloroformate, which reacts with another phenoxide, forming:

<math>(NaOC6H4)2CMe2 + COCl2 → 1/n [OC(OC6H4)2CMe2]n + 2 NaCl</math>

IMPORTANT: Please note that creation by this method is very dangerous for home synthesis. Bisphenol A is an endocrine disruptor, and should be handled carefully. Phosgene should be considered especially dangerous. Consider that it was used as a chemical weapon during World War I. It is only detected by smell far above the danger threshold, and is slow acting so damage may not be readily apparent.

Various alternatives to using Bisphenol A exist. other diols can be substituted, with varying differences to overall properties. 1,1-bis(4-hydroxyphenyl)cyclohexane and dihydroxybenzophenone have been sued effectively, ad well as tetramethylcyclobutanediol which has not been shown to be an endocrine disruptor.

A method more suitable to laboratory synthesis is transesterification of bisphenol A (or some other suitable diol) and diphenyl carbonate:

<math>(HOC_6H_4)2CMe_2 + (C_6H_5O)2CO → 1/n [OC(OC_6H_4)2CMe_2]n + 2 C_6H_5OH</math>

Crafting

Polycarbonate has a high impact-resistance, but very low scratch resistance. Areas likely to be under high wear such as sunglasses and automotive components tend to have a scratch resistant coating applied.

Polycarbonate's glass transition temperature is <math>150^\circ C (302^\circ F)</math>, softening gradually above this point, and flowing freely at temperatures above <math>300^\circ C (572^\circ F)</math>. However, in it's free-flowing state the material is also a sheer-thickening fluid which temporarily crystallizes when exposed to high sheer forces, making injection molding techniques difficult. This can be partially alleviated by keeping tools and molds at high temperatures (above<math>90^\circ C</math>). Products without this tend to contain flaws. The higher the molecular mass of the sample, the better the quality, but the higher the processing difficulty.

Polycarbonate differs significantly from most thermoplastics in that it can undergo large deformations without significant loss of structural integrity. As a result, it can be formed using conventional sheet metal techniques such as bending on a sheet metal brake.

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